1. Earth Science Chapter 17: Plate Tectonics

2. Section 1 – Drifting Continents

3. 17.1 – Essential Questions
What are the lines of evidence that led Wegener to suggest that Earth’s continents have moved?
How does evidence of ancient climates support continental drift?
Why was continental drift not accepted when it was first proposed?

4. Main Idea
The shape and geology of the continents suggests that they were once joined together.

5. Early Observations
With the exception of events such as earthquakes, volcanic eruptions, and landslides, most of Earth’s surface appears to remain relatively unchanged during the course of a human lifetime.
On the geologic time scale, however, Earth’s surface has changed dramatically.

6. Early Observations
In the late 1500s, Abraham Ortelius, a Dutch cartographer (map maker), noticed the apparent fit of continents on either side of the Atlantic Ocean.
He proposed that North America and South America had been separated from Europe and Africa by earthquakes and floods.

7. Early Observations
The first time that the idea of moving continents was proposed as a scientific hypothesis was in the early 1900s.
In 1912, German meteorologist Alfred Wegener presented his ideas about continental movement to the scientific community.

8. Continental Drift
Wegener developed a hypothesis that he called continental drift.
He proposed that Earth’s continents had once been joined in a single landmass, a supercontinent called Pangaea (Meaning All Lands), that broke apart about 200 mya (million years ago) and sent the continents adrift.

10. Continental Drift – Evidence from Rock Formations
Wegener observed that many layers of rocks in the Appalachian Mountains in the United States were identical to layers of rocks in similar mountains in Greenland and Europe.
These similar groups of rocks, older than 200 million years, supported Wegener’s idea that the continents had once been joined.

11. Continental Drift – Evidence from Fossils
Wegener gathered evidence of the existence of Pangaea from fossils.
Similar fossils of animals and plants that once lived on or near land had been found on widely separated continents.

12. Evidence from Fossils

13. Continental Drift – Climatic Evidence
Fossils of the plant Glossopteris had been found on many parts of Earth, including South America, Antarctica, and India.
Wegener reasoned that the area separating these fossils was too large to have had a single climate.
Wegener argued that because Glossopteris grew in temperate climates, the places where the fossils had been found had been closer to the equator.
This led him to conclude that the rocks containing these fossil ferns had once been joined.

14. Continental Drift – Climatic Evidence
Coal forms from the compaction and decomposition of accumulations of ancient swamp plants.
Wegener used the existence of coal beds in Antarctica to conclude that Antarctica must have been much closer to the equator sometime in the geologic past.

15. Continental Drift – Climatic Evidence
Glacial deposits nearly 300 million years old on several continents led Wegener to propose that these landmasses might have once been joined and covered with ice. The extent of the ice is shown in white.

16. A Rejected Notion Two unanswered questions—
Although Wegener had compiled an impressive collection of data, the hypothesis of continental drift was not accepted by the scientific community.
Two unanswered questions—
What forces could cause the movement?
How could continents move through solids?
Main reasons that continental drift was rejected.

17. A Rejected Notion
It was not until the early 1960s, when new technology revealed more evidence about how continents move, that scientists began to reconsider Wegener’s ideas.

18. Plate Tectonics Video

19. 17.2 – Seafloor Spreading

20. 17.2 Essential Questions
What evidence led to the discovery of seafloor spreading?
What is the significance of magnetic patterns on the seafloor?
How is the process of seafloor spreading explained?

21. Oceanic crust forms at ocean ridges and becomes part of the seafloor.
Main Idea
Oceanic crust forms at ocean ridges and becomes part of the seafloor.

22. Mapping the Ocean Floor
Until the mid-1900s, many scientists thought that the ocean floors were essentially flat and that oceanic crust was unchanging and was much older than continental crust.
Advances in technology during the 1940s and 1950s (Sonar & Magnetmeter) showed that all of these widely accepted ideas were incorrect.
Harry Hess – Proposed Seafloor Spreading

23. Mapping the Ocean Floor
One technological advance that was used to study the ocean floor was the magnetometer, a device that can detect small changes in magnetic fields.
Towed behind a ship, it can record the magnetic field generated by ocean floor rocks.
Developments in sonar technology enabled scientists to measure water depth and map the topography of the ocean floor.

24. Ocean-Floor Topography
Using the maps made from data collected by sonar and magnetometers, scientists discovered that vast, underwater mountain chains called ocean ridges run along the ocean floors around Earth much like seams on a baseball.
Maps generated with sonar data revealed that underwater mountain chains had counterparts called deep-sea trenches.

26. Ocean-Floor Topography
The deepest trench, the Mariana Trench, is more than 11 km (6.3 miles) deep.
Mount Everest, the world’s tallest mountain, stands at 9 km (5.9 miles) above sea level, and could fit inside the Mariana Trench with six Empire State buildings stacked on top.

27. Ocean Rocks & Sediments
The ages of the rocks that make up the seafloor vary across the ocean floor, and these variations are predictable.
The age of oceanic crust increases with distance from a ridge.
Ocean-floor sediments are typically a few hundred meters thick.
Large areas of continents, on the other hand, are blanketed with sedimentary rocks that are as much as 20 km thick.

28. Ocean Rocks & Sediments
Observations of ocean-floor sediments revealed that, like the age of ocean crust, the thickness of ocean-floor sediments increases with distance from an ocean ridge.

29. Magnetism
Earth has a magnetic field generated by the flow of molten iron in the outer core. This field is what causes a compass needle to point to the North.
A magnetic reversal happens when the flow in the outer core changes, and Earth’s magnetic field changes direction.

30. Magnetism
A magnetic field that has the same orientation as Earth’s present field is said to have normal polarity. A magnetic field that is opposite to the present field has reversed polarity.

31. Magnetic Polarity Time Scale
Paleomagnetism is the study of the history of Earth’s magnetic field.
When lava solidifies, iron-bearing minerals such as magnetite crystallize.
As they crystallize, these minerals behave like tiny compasses and align with Earth’s magnetic field.

33. Magnetic Polarity Time Scale
Periods of normal polarity alternate with periods of reversed polarity.
Long-term changes in Earth’s magnetic field, called epochs, are named as shown here.
Short-term changes are called events.

34. Magnetic Symmetry
Regions of normal and reverse polarity form a series of stripes across the ocean floor parallel to the ocean ridges.
The ages and widths of the stripes match from one side of the ridges to the other.

35. Magnetic Symmetry
By matching the magnetic patterns on the seafloor with the known pattern of magnetic reversals on land, scientists were able to determine the age of the ocean floor from magnetic recording and to create isochron maps of the ocean floor.

36. Magnetic Symmetry
An isochron is an imaginary line on a map that shows points that have the same age—that is, they formed at the same time.

37. Visualizing Seafloor Spreading
Data from topographic, sedimentary, and paleomagnetic research led scientists to propose seafloor spreading.

38. Seafloor Spreading
Seafloor spreading is the theory that explains how new ocean crust is formed at ocean ridges and destroyed at deep-sea trenches.

39. Seafloor Spreading
During seafloor spreading, magma, which is hotter and less dense than surrounding mantle material, is forced toward the surface of the crust along an ocean ridge.
As the two sides of the ridge spread apart, the rising magma fills the gap that is created.
When the magma solidifies, a small amount of new ocean floor is added to Earth’s surface.

40. Seafloor Spreading
As spreading along an ocean ridge continues, more magma is forced upward and solidifies.
The cycle of spreading and the intrusion of magma continues the formation of ocean floor, which slowly moves away from the ridge.

42. Seafloor Spreading Videos

43. 17.3 – Plate Boundaries

44. 17.3 Essential Questions
How does the movement of Earth’s tectonic plates result in many geologic features?
What are the three types of plate boundaries and the features associated with each?
What are the processes associated with subduction zones?

45. 17.3 Main Idea
Volcanoes, mountains, and deep-sea trenches form at the boundaries between the plates.

46. Theory of Plate Tectonics
Tectonic plates are huge pieces of crust and rigid upper mantle that fit together at their edges to cover Earth’s surface.

47. Theory of Plate Tectonics
Plate tectonics is the theory that describes how tectonic plates move and shape Earth’s surface.
They move in different directions and at different rates relative to one another, and they interact with one another at their boundaries.

48. Divergent Boundaries
Regions where two tectonic plates are moving apart are called divergent boundaries.
Most divergent boundaries are found along the seafloor in rift valleys.
The formation of new ocean crust at most divergent boundaries accounts for the high heat flow, volcanism, and earthquakes associated with these boundaries.

49. Divergent Boundaries
Some divergent boundaries form on continents. When continental crust begins to separate, the stretched crust forms a long, narrow depression called a rift valley.

50. Convergent Boundaries
At convergent boundaries, two tectonic plates are moving toward each other.
When two plates collide, the denser plate eventually descends below the other, less-dense plate in a process called subduction.
There are three types of convergent boundaries, classified according to the type of crust involved. The differences in density of the crustal material affect how they converge

51. Convergent Boundaries
In the oceanic-oceanic convergent boundary, a subduction zone is formed when one oceanic plate, which is denser as a result of cooling, descends below another oceanic plate. The process of subduction creates an ocean trench.
In an oceanic-oceanic convergent boundary, water carried into Earth by the subducting plate lowers the melting temperature of the overlying mantle, causing it to melt.
The molten material is less dense so it rises back to the surface, where it often erupts and forms an arc of volcanic islands that parallel the trench.

52. Convergent Boundaries
When an oceanic plate converges with a continental plate, the denser oceanic plate is subducted. Oceanic-continental convergence produces a trench and volcanic arc.
The result is a mountain range with many volcanoes.

53. Convergent Boundaries
Continental-continental boundaries form when two continental plates collide, long after an oceanic plate has converged with a continental plate.
This forms a vast mountain range, such as the Himalayas.

54. Transform Boundaries
A region where two plates slide horizontally past each other is a transform boundary.

55. Transform Boundaries
Transform boundaries are characterized by long faults, sometimes hundreds of kilometers in length, and by shallow earthquakes.
Most transform boundaries offset sections of ocean ridges.
Sometimes transform boundaries occur on continents.

56. 17.4 – Causes of Plate Motions

57. How is the process of convection explained?
17.4 Essential Questions
How is the process of convection explained?
How is convection in the mantle related to the movements of tectonic plates?
What are the processes of ridge push and slab pull?

58. Convection currents in the mantle cause plate motions.
Main Idea
Convection currents in the mantle cause plate motions.

59. Convection
Many scientists now think that large-scale motion in the mantle—Earth’s interior between the crust and the core—is the mechanism that drives the movement of tectonic plates.

60. Convection Currents
Convection is the transfer of thermal energy by the movement of heated material from one place to another.
The cooling of matter causes it to contract slightly and increase in density.
The cooled matter then sinks as a result of gravity.
Warmed matter is then displaced and forced to rise.
This up-and-down flow produces a pattern of motion called a convection current.

61. Convection Currents
Water cooled by the ice cube sinks to the bottom where it is warmed by the burner and rises. The process continues as the ice cube cools the water again.

62. Convection in the Mantle
Convection currents develop in the mantle, moving the crust and outermost part of the mantle and transferring thermal energy from Earth’s interior to its exterior.

64. Plate Movement
The rising material in a convection current spreads out as it reaches the upper mantle and causes both upward and sideways forces, which lift and split the lithosphere at divergent plate boundaries.

65. Plate Movement
The downward part of a convection current occurs where a sinking force pulls tectonic plates downward at convergent boundaries.

66. Push and Pull
Ridge push is the tectonic process associated with convection currents in Earth’s mantle that occurs when the weight of an elevated ridge pushes an oceanic plate toward a subduction zone.

67. Push and Pull
Slab pull is the tectonic process associated with convection currents in Earth’s mantle that occurs as the weight of the subducting plate pulls the trailing lithosphere into a subduction zone.